Conventional telephone systems operate in full duplex mode. This means that both parties linked by the telephones can speak at the same time. Many radio systems operate in a mode known as half duplex mode which means that only person at an end can talk at a time. The direction of the communication is controlled by a "press to talk button" which is activated by the party wishing to speak. In order to avoid confusion, the party that is talking must indicate verbally that the user is relinquishing the channel by saying "over". Examples of such systems include air to ground radios, HF radios and many portable radio systems. The reason for the popularity of half duplex despite its inconvenience is that it uses only one frequency, an important consideration where frequencies are at a premium. One disadvantage of a manually controlled half duplex communications system is that it is difficult to interface a half duplex radio system to a conventional telephone. In addition, normal conversation which allows one party to interrupt the other is impeded by the press to talk operation. This is especially annoying for persons accustomed to the normal telephone system which is full duplex.
A channel which has only sufficient capacity to carry voice information in one direction generally cannot be used to provide a two way voice link simultaneously.
Full duplex can be achieved if the two ends of the link operate in what is known as Time Division Duplex (TDD) mode. The channel is allocated 50 percent of the time to transmit information in each of the two directions. That is, the radio channel is divided into time slots of T/2 seconds, each end transmitting in alternate time slots. When one end is transmitting, the opposite end is receiving the information. In this way one frequency can be used for information flow in both directions resulting in a full duplex link.
One such system is the cordless telephone system known as CT2 in which the voice is digitized by an ADPCM codec to 32 kbits/sec. The transmission rate of the data through the radio channel is about 64 kbits/sec, so that in each T/2 second time slot 64 kbits of information are transmitted in each direction. In addition some additional bits are introduced into the data to accommodate synchronization and other "house keeping" information needed to keep the timing between the two ends of the link aligned. The data received within the T/2 time slot at 64 kbits/sec is expanded to form a uniform 32 kbit/sec bit stream after house keeping information is stripped off, and is applied to the codec to be converted to voice. For the TDD system to work the channel must be capable of carrying more than twice the data rate needed for one voice codec.
For systems such as HF radio, and VHF AM radios used for air traffic control the maximum rate that can be carried reliably within the assigned band is not sufficient for transmission of voice using time division duplex as described in the previous paragraph with the present state of the speech coding and data transmission technology. For example, HF radios have the capability to carry data reliably at 2.4 Kbits/sec and a codec which can encode high quality voice at 1.2 kbits/sec does not presently exist.
A system which allows a full duplex voice communication such as a telephone conversation over the same narrow-band radio channel uses time compression of the voice signal and analog time division multiplex of the channel, is described in U.S. Pat. No. 5,355,363 dated Oct. 11, 1994, invented by Takahashi et al. In that system, a segment of analog voice with duration T seconds is converted into a segment of analog signal with duration T/2 seconds. The encoded version of the (analog) voice signal occupies a bandwidth which meets the requirements of the regulatory bodies, so but in order to contain the signal with the channel, at least half of the voice bandwidth is eliminated. This substantially degrades the fidelity of the signal.
In the system described in the patent, the transmit and receive timing for the channel is determined by a synchronization signal that is transmitted in the same channel the analog signal, which of course limits the channel time, restricting further the frequencies of the signal that can be transmitted, and the resulting fidelity of the voice signal.